1. Field of the Invention
The present invention relates to an image heating apparatus, which heats an image formed on a recording material. By providing the image heating apparatus, it is possible to obtain a fixing apparatus for fixing an unfixed image on a recording material, and a gloss improving apparatus for improving gloss of an image by re-heating an image fixed on a recording material. The image heating apparatus is used in an image forming apparatus such as a copying machine, a printer, a facsimile, and a composite machine having a plurality of functions thereof.
2. Related Background Art
Up to now, in such the image forming apparatus, as a fixing method of fixing an unfixed toner image on a recording material, a thermal fixing method in which an unfixed toner image is heated and fused to be fixed on the recording material is generally used in view of safety and excellent fixing property.
In particular, in view of excellent thermal efficiency, easiness of down-sizing, and the like, widely used is a heat roller method in which an unfixed toner image formed on a recording material is heated and pressurized to be thermally-fixed on a fixing area in which a heat roller and a pressure roller are in pressure contact with each other.
A heat-roller-type fixing apparatus uses a fixing roller provided with a heater therein and a pressure roller which is opposed to and brought into pressure contact with the fixing roller, thereby introducing a recording material into a fixing nip portion located between the pair of rollers, to be passed through the pair of rollers. Thus, an unfixed toner image formed and carried on a surface of the recording material is fixed by heat and pressure.
In recent years, a film-heating-type fixing apparatus is put into practical use from the viewpoint of quick-start ability and energy-saving ability.
In the film-heating-type fixing apparatus, a heat-resistant film (hereinafter, referred to as “fixing film”) is sandwiched between a ceramic heater serving as a heating member and a pressure roller serving as a pressure member, thereby forming a fixing nip portion. Then, a recording material on which an unfixed toner image is formed and carried is introduced between the fixing film and the pressure roller of the fixing nip portion, thereby being nipped and transported together with the fixing film. As a result, the unfixed toner image is fixed on the surface of the recording material by contact pressure of the fixing nip portion while being supplied with heat of the ceramic heater through the fixing film.
In the film-heating-type fixing apparatus, it is possible to constitute an on-demand-type apparatus by using a member having a lower heat capacity for the ceramic heater and the film, and it is sufficient that the ceramic heater serving as a heat source is energized only at the time of executing image formation to heat the ceramic heater up to a predetermined fixing temperature. Therefore, the film-heating-type fixing apparatus has advantages in that a waiting time between power-on of an image forming apparatus and a time point of being in a state where image formation is ready to be executed is short (i.e., quick-start ability), power consumption in a standby state is significantly reduced (i.e., power-saving), and the like.
In such the film-heating-type fixing apparatus, a conventional feed-back type power control is performed. In this control, based on temperature detected by, for example, a temperature detecting means which is provided by being bonded or the like to a back surface of the ceramic heater, electric energy applied to the heater is controlled by a method such as a proportional control to thereby keep the heater at constant temperature.
In the heat-roller-type or film-heating-type fixing apparatus described above, there is a problem of temperature rise of a non-sheet passing portion at a time of continuous supply of recording materials having a narrower width (hereinafter, referred to as “small-size sheet”) than a recording material having a maximum sheet passing width (hereinafter, referred to as “maximum-size sheet”).
Recording materials having a variety of sizes (i.e., widths) pass a fixing area. The fixing area through which the recording materials pass is called a sheet passing area, and a fixing area other than the sheet passing area is called a non-sheet passing area. In addition, a surface portion of the heat roller which passes the sheet passing area at a time of rotation is called a sheet passing area passing surface, and a surface portion of the heat roller which passes the non-sheet passing area at a time of rotation is called a non-sheet passing area passing surface.
When the maximum-size sheet is passed to have an image fixation thereto, it is possible to obtain a temperature distribution in which temperature of the surface of the heat roller is substantially the same over the whole length of the fixing area. However, when the small-size sheets are continuously supplied to have an image fixation thereto, temperature of the non-sheet passing area passing surface of the heat roller is excessively raised. This is because, when the small-size sheets are continuously supplied, heat is not drawn by a sheet in the non-sheet passing area through which a sheet does not pass, so heat is accumulated in the non-sheet passing area.
Accordingly, a fixing device described in JP S60-136779 A has a structure in which the non-sheet passing portion described above is cooled by a cooling fan.
To be specific, in the fixing device described in JP S60-136779 A, a temperature sensor, which detects the temperature of the non-sheet passing portion is provided, and the cooling fan is turned on when the temperature detected by the temperature sensor becomes 210° C. or higher. Then, the cooling fan is turned off when the temperature detected by the temperature sensor becomes lower than 210° C.
However, in the fixing device described in JP 60-136779 A, there is a fear that the following problem may be caused.
In other words, because the fixing device has the structure in which the cooling fan is turned on while the temperature detected by the temperature sensor is 210° C. or higher, the cooling fan is maintained to be turned on even when an image forming job is completed.
Accordingly, even when energization to the heater is stopped in accordance with the end of the job to naturally cool the non-sheet passing portion, the cooling fan is continuously operated after the job is completed, which leads to unnecessary energy consumption.
Further, it is preferable that the film be rotationally driven during the operation of the cooling fan to prevent uneven cooling, so there is a fear that a life span of the film is shortened due to the continuous operation of the cooling fan after the job is completed.